feat: documentation website using Vitepress

Adds documentation website using https://vitepress.dev.

Some decisions are questionable, for example creation of a symbolic link
for the README in the `docs/`. I wanted to preserve the ability to read
docs using Github. Maybe not needed.

Another issue, is the page outline for the `NixOS` chapter is too
narrow.

But should serve as a scaffold/basis for doing it properly.

docs/.vitepress/config.ts

@@ -0,0 +1,43 @@
+import { defineConfig } from "vitepress";
+
+// https://vitepress.dev/reference/site-config
+export default defineConfig({
+	title: "Headplane",
+	description: "Documentation",
+	// base: "https://headplane.net",
+	ignoreDeadLinks: ["/docs/Integrated-Mode", "/docs/Simple-Mode"],
+	themeConfig: {
+		// https://vitepress.dev/reference/default-theme-config
+		nav: [{ text: "Home", link: "/" }],
+
+		sidebar: [
+			{
+				text: "Chapters",
+				items: [
+					{ text: "Getting Started", link: "/README" },
+					{ text: "Configuration", link: "/Configuration" },
+					{ text: "Bare-Metal Mode", link: "/Bare-Metal" },
+					{ text: "Integrated Mode", link: "/Integrated-Mode" },
+					{ text: "Simple Mode", link: "/Simple-Mode" },
+					{ text: "Nix", link: "/Nix" },
+					{ text: "NixOS", link: "/NixOS-options" },
+					{ text: "Security", link: "/SECURITY" },
+					{ text: "Contributing", link: "/CONTRIBUTING" },
+					{ text: "Changelog", link: "/CHANGELOG" },
+				],
+			},
+		],
+
+		socialLinks: [
+			{ icon: "github", link: "https://github.com/tale/headplane" },
+		],
+
+		lastUpdated: {
+			text: "Updated at",
+			formatOptions: {
+				dateStyle: "full",
+				timeStyle: "medium",
+			},
+		},
+	},
+});